1. Field of the Invention
The present invention relates to an image-forming apparatus such as a copier, a facsimile and a printer, in particular an image-forming apparatus comprising a latent image carrier which forms a latent image on its surface, a developing means to render this latent image into a visible image as a toner image by adhesion of toner, a transfer means which transfers the toner image formed on this latent image carrier to a transfer material without using an electrostatic transfer method, and a fixing means which fixes the transferred toner image on the transfer material by heat and pressure. The present invention also relates in particular to an image-forming method using this image-forming apparatus.
2. Description of the Related Art
In the prior art, in this kind of image-forming apparatus, an electrophotographic image-forming method is used wherein an image is formed in a large number of steps, such as a latent image-forming step, a developing step, a transfer step and a fixing step. For example, in the case of a copier, a document is converted into electrical signals by a scanner or an optical system. In the case of a printer, electrical signals are directly input. An electrostatic latent image is formed on a charged photoconductor on which an optical image is applied by a patterned irradiation of a laser beam or the like, the patterned irradiation being made by the electric signals. A charged colored fine powder such as a toner is made to adhere to the latent image in a developing step. This is then electrostatically transferred to a transfer paper in a transfer step. Recently, in the color-printing field, various methods are being used to transfer a three- or four-colored toner image to an intermediate transfer object. The toner is then melted and fixed on the transfer paper to form the image.
In each of the aforesaid steps, deterioration of the image (including the latent image) occurs. It is particularly well-known that deterioration of the image is especially large during developing, transfer and fixing.
In the developing step, as toner adheres electrostatically to the photoconductor latent image due to the electric field surrounding the toner on the photoconductor, developing takes place over a larger area than the latent image or becomes blurred due to scratching of the carrier, and image deterioration of the electrostatic latent image occurs. In recent years, this has been improved by the fineness and sphericity of toner and the fineness of the carrier, but it still does not give sufficient image quality.
In the transfer step, a transfer material transported in synchronism with the photoconductor to which the developed toner adheres is brought into contact, and is electrostatically transferred from the photoconductor to a transfer paper due to the electric field. However, dust and blurring occur electrostatically during intimate contact with the transfer paper before and after this transfer step, and image deterioration increases.
Also in the fixing step, deterioration may occur due to the spread of the toner image from melting of toner in the step wherein toner is fused with the transfer paper. Further, when there is scatter in the amount of toner deposit on the transfer paper, scatter in the dot diameter or line width may increase, and deterioration may occur.
Of the aforesaid image deteriorations, the image deterioration in the transfer step is particularly large. In this regard, various techniques of performing the transfer step and fixing step simultaneously have been disclosed.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 55-87156, a method of performing transfer and fixing on the transfer paper simultaneously using an amorphous silicone photoconductor and a heating fixing roller is disclosed. JP-A No. 06-175512 discloses a method of performing transfer and fixing simultaneously by thermal energy using a polymerization toner.
In these methods, as it is possible to transfer the toner image without a transfer electric field, a toner image of low resistance toner can also be transferred well, while avoiding image deterioration due to transfer dust.
To obviate the adverse effect of heat on the system, a method which does not heat but performs simultaneous transfer and fixing only by pressure has been proposed. For example, in JP-A No. 03-186879, a method of fixing at a pressure of 1470-2450 N/cm2 is disclosed. In JP-A No. 05-216354 or JP-A No. 06-35341, a method of carrying out simultaneous transfer and fixing of toner image on a photoconductor by a transfer fixing apparatus facing the photoconductor, using an amorphous silicone photoconductor and capsule toner, is disclosed.
In JP-A No. 07-5776, a method of applying a transfer bias to a pressure roller using an amorphous silicone photoconductor, and using capsule toner as toner, is disclosed.
Many techniques for using capsule toner are disclosed for example in JP-A No. 05-107796 and JP-A No. 06-230599.
However, in the simultaneous thermal transfer and fixing method which performs transfer and fixing simultaneously by heat disclosed in JP-A No. 55-87156, since a heating body touches the photoconductor, the circumference of the photoconductor reaches a temperature higher than the melting point of the toner. For this reason, a large stress acts on the toner at the photoconductor, developing apparatus and cleaning apparatus, and presents a major problem in practical applications. Even if a cooling device is provided, damage to the photoconductor, the toner on the photoconductor and the developing apparatus, is unavoidable.
In the simultaneous pressure transfer and fixing method as disclosed in JP-A No. 03-186879 in which transfer and fixing are simultaneously performed by pressure alone, it is necessary to apply a large pressure from the viewpoint of transfer fixability. For example, in JP-A No. 05-216354, it is stated that the transfer fixing pressure is preferably 980 N/cm2 or more. This requires a larger and heavier apparatus due to mechanisms and transfer paper transportation, and leads to fixing creases in the transfer paper and image broadening. Due to these problems, simultaneous transfer and fixing only by pressure are not in practical use.
Moreover, in the inventions disclosed in JP-A No. 05-216354 and JP-A No. 07-5776, the toner image can be transferred to the recording medium by pressurizing and crushing a capsule toner in a transfer position without a transfer electric field. However, it is difficult to reconcile good developing and fixing properties for the capsule toner, and there is the drawback of high cost.